A general principle of hologram preparation are described in certain literatures and publishings, such as Junpei Tsujiuchi, “Holographic Display”, published by Sangyo Tosho, chapter 2. In such principle, one of two coherent laser beams irradiates an object to be recorded, and a hologram recording material is placed in a position capable of receiving a total reflected light therefrom. The hologram recording material is irradiated, in addition to the reflected light from the object, directly by the other coherent light beam which arrives without going through the object. The reflected light from the object is called an object light, while the light directly irradiating the recording material is called a reference light, and interference fringes of the reference light and the object light are recorded as image information. Then, when a light (reproducing illumination light) same as the reference light irradiates the processed recording material, it is so diffracted by the hologram as to reproduce a wave front of the reflected light when it at first reaches the recording material from the object at the recording, whereby an object image, substantially same as the real image of the object, can be viewed three-dimensionally.
A hologram prepared by introducing the reference light and the object light from a same direction into the hologram recording material is called a transmission hologram. The interference fringes are formed perpendicularly or almost perpendicularly to the film surface of the recording material, with a pitch of 1000-3000 fringes per millimeter.
On the other hand, a hologram prepared by introducing the reference light and the object light mutually from opposite sides of the hologram recording material is called a reflective hologram. The interference fringes are formed parallel or almost parallel to the film surface of the recording material, with a pitch of 3000-7000 fringes per millimeter.
The transmission hologram can be prepared by a known method described for example in JP-A-6-43634. Also the reflective hologram can be prepared by a known method described for example in JP-A-2-3082 and JP-A-3-50588.
Also, a hologram having a film thickness sufficiently larger than the pitch of the interference fringes (usually a film thickness of about 5 times or more of the pitch of the interference fringes or of about 1 μm or larger) is called a volume hologram.
On the other hand, a hologram having a film thickness of 5 times or less of the pitch of the interference fringes or of about 1 μm or less is called a planar or surface type hologram.
Also a hologram which records interference fringes by an absorption of a dye or silver is called an amplitude hologram, and a hologram which performs a recording by a surface relief formation or a refractive index modulation is called a phase hologram. The amplitude hologram is unfavorable in the efficiency of light utilization because the light absorption significantly decreases a diffraction efficiency or a reflection efficiency of light, and a phase hologram is advantageously employed.
A volume phase hologram can modulate a phase of light without light absorption, by forming a plurality of interference fringes different in the refractive index, instead of the optical absorption, in the hologram recording material.
In particular, the volume phase hologram of reflective type is also called Lippman hologram, and is capable, by a wavelength-selective reflection by Bragg's diffraction, of achieving a full-color image, a reproduction with white light and a high resolution, whereby a high-resolution full-color three-dimensional display can be realized.
Also utilizing the wavelength-selective reflection, it is widely applied to holographic optical elements (HOE) such as a head-up display (HUD) for an automobile, a pickup lens for an optical disk, a head-mount display, a color filter for a liquid crystal display, and a reflecting plate for a reflective liquid crystal display.
In addition, it is commercially practiced or investigated in a lens, a diffraction grating, an interference filter, a coupler for an optical fiber, a photodeflector for a facsimile, a window pane material for a building and the like.
On the other hand, the recent progress of so-called information society rapidly promotes pervasiveness of networks such as Internet and the high-vision TV. Also the broadcasting of the HDTV (high definition television) is planned shortly, and there is anticipated an increasing demand, also in consumer applications, for a high-density recording medium capable of recording image information of 100 GB or more inexpensively and easily.
Also the progress of the computers toward a higher capacity is promoted, also in business applications such as computer backup or broadcasting backup, a demand for an ultra high-density recording medium capable of inexpensively recording information of a large capacity of about 1 TB or more, at a high speed.
In such trends, a compact and inexpensive optical recording medium, being flexible and capable of random access, is considered promising in comparison with a magnetic tape incapable of random access or a hard disk which is not exchangeable and often causes failures. However, in an existing two-dimensional optical recording medium such as DVD-R, a storage capacity is limited to about 25 GB on one side at maximum because of the physical principle even if a short wavelength is employed for recording and reproduction, and a sufficiently large recording capacity capable of meeting the future demand cannot be anticipated.
Therefore, as an ultimate high-density recording medium, a three-dimensional optical recording medium which performs recording in the direction of thickness is recently attracting attention. Promising candidates for such a purpose include a method utilizing a two-photon absorbing material and a method utilizing holography (interference), and the volume phase hologram recording material is being investigated actively as a three-dimensional optical recording medium (holographic memory).
A holographic memory utilizing a volume phase hologram recording material records a plurality of two-dimensional digital information (also called signal light) utilizing a spatial light modulator (SLM) such as DMD or LCD, instead of an object light reflected from a three-dimensional object. At the recording, there are performed multiplex recording such as an angular multiplexing, a phase multiplexing, a wavelength multiplexing or a shift multiplexing, thereby realizing a capacity as high as 1 TB. Also a readout operation is performed with a CCD or a CMOS sensor, and a transfer rate as high as 1 Gbps can be realized by parallel writing and readout.
However, requirements for the hologram recording material for such holographic memory are even stricter than those for application for a three-dimensional display or an HOE, as indicated in the following:    (1) a high sensitivity;    (2) a high resolution;    (3) a high diffraction efficiency of hologram;    (4) a dry and rapid processing at the recording;    (5) an ability for multiplex recording (a wide dynamic range);    (6) a low shrinkage after recording; and    (7) a satisfactory storability of hologram.
In particular, the requirement of (1) high sensitivity is chemically contradictory to those of (3) high diffraction efficiency, (4) dry process, (6) low shrinkage after recording and (7) satisfactory storability, and it is difficult to satisfy these requirements at the same time.
The known recording materials for the volume phase hologram include, as a write-once type, a bichromate gelatin system, a bleached silver halide system and a photopolymer system, and, as a rewritable type, a photorefractive system and a photochromic polymer system.
However, among these known recording materials for the volume phase hologram, particularly for the application for a high sensitivity optical recording medium, there has not been known a material that can satisfy all the requirements and an improvement is being desired.
More specifically, for example the bichromate gelatin system has the advantages of a high diffraction efficiency and low noise characteristics, but is associated with drawbacks of an extremely poor storability and a low sensitivity and requiring a wet process, thus being unsuitable for use in the holographic memory.
The bleached silver halide system has an advantage of a high sensitivity, but requires a wet process with a cumbersome bleaching process and is associated with drawbacks of a large scattering and a poor light fastness, thus being generally unsuitable for use in the holographic memory.
The photorefractive material has an advantage of being rewritable, but has drawbacks of requiring an application of a high electric field at the recording and of a poor storability of record.
Also a photochromic polymer system represented by an azobenzene polymer material has an advantage of rewritability, but is associated with drawbacks of an extremely low sensitivity and a poor storability of record. For example WO 97/44365 A1 discloses a rewritable hologram recording material utilizing an anisotropy in refractive index and an alignment control in an azobenzene polymer (photochromic polymer), but such material is far from practical use because of an extremely low sensitivity as it has a low quantum yield in the isomerization of azobenzene and involves an alignment control, and also of a poor storability of record as a trade-off of rewritability.
Among these, a dry process photopolymer system disclosed in JP-A-6-43634, JP-A-2-3082 and JP-A-3-50588 employs a basic composition of a binder, a radical polymerizable monomer and a photopolymerization initiator and creates a difference in the refractive index by employing a compound having an aromatic ring, chlorine or bromine in either of the binder and the radical polymerizable monomer in order to increase the refractive index modulation, whereby the polymerization proceeds with a concentration of the monomer in light parts of the interference fringes and a concentration of the binder in dark parts of the interference fringes, formed at the holographic exposure, thereby genrating a difference in the refractive index. Therefore, it can be considered as a relatively practical system in which a high diffraction efficiency and a dry process can be realized at the same time.
However, such a system involves drawbacks of a poor sensitivity of about 1/1000 in comparison with the bleached silver halide system, a necessity for a heat fixing treatment as long as almost 2 hours for improving the diffraction efficiency, an inhibition by oxygen of the involved radical polymerization, and a shrinkage of the recording material after exposure and fixation thereby resulting in a change in diffraction wavelength and angle at the reproducing operation, and is hardly usable in the application for a holographic memory.
In general, in comparison with a radical polymerization, a cationic polymerization, particularly a ring-opening cationic polymerization of an epoxy compound or the like, shows a smaller shrinkage after polymerization, also is not subjected to an inhibition of polymerization by oxygen, and provides a film with a rigidity. It is therefore pointed out that a cationic polymerization is more suitable for the application as a holographic memory.
For example, JP-A-5-107999 and JP-A-8-16078 disclose a hologram recording material employing a cationic polymerizable compound (monomer or oligomer) instead of a binder, and further combining a sensitizing dye, a radical polymerization initiator, a cationic polymerization initiator and a radical polymerizable compound.
Also JP-T-2001-523842 and JP-T-11-512847 disclose a hologram recording material not utilizing a radical polymerization but employing a sensitizing dye, a cationic polymerization initiator, a cationic polymerizable compound and a binder only.
However, these cationic polymerization systems, though showing an improvement in the shrinkage rate in comparison with the radical polymerization system, show a decreased sensitivity as a trade-off, which will lead to a major drawback in the transfer rate in the practice. These systems also show a lowered diffraction efficiency, which will lead to drawbacks in an S/N ratio and a multiplex recordability.
As the photopolymer system involves a material transfer, in the application to a holographic memory, there results a dilemma as explained in the foregoing that an improved storability and a reduced shrinkage lead to a decreased sensitivity (cationic polymerization system) and an improved sensitivity leads to a loss in the storability and the shrinkage (radical polymerization system).
Also in order to increase the recording density of a holographic memory, there is required multiplex recording of 50 times or more and preferably 100 times or more, and, in the photopolymer system utilizing a polymerization process involving a material transfer for recording, the recording speed becomes lower in a latter stage of the multiplex recording where a large proportion of the compound has already polymerized in comparison with an initial stage of the multiplex recording, and it is practically difficult to regulate the exposure amount and to obtain a wide dynamic range by controlling such difference in the recording speed.
The aforementioned trade-off among a high sensitivity, a satisfactory storability, a low shrinkage and a dry process, and the limitation in the multiplex recordability (high recording density) are difficult to avoid, because of the physical laws, within the related-art photopolymer system. It is also difficult to meet the requirements for the holographic memory in the silver halide system, particularly in principle in achieving dry process.
Therefore, in order to apply a hologram recording material to a holographic memory, it is strongly desired to develop a totally novel recording method capable of fundamentally resolving such drawbacks, particularly attaining a high sensitivity, a low shrinkage, a satisfactory storability, a dry processability and a multiplex recordability at the same time.
Also in the related-art applications of the display hologram or the holographic optical element, a film thickness of the hologram recording material can be several tens of micrometers or less, but, in the holographic memory, there is required a film thickness of 100 μm or larger, and as large as 500 μm to 1 mm in certain cases, in order to attain a multiplicity (recording density). For increasing the multiplicity, the hologram recording light has to be transmitted much even in such film thickness, but the known sensitizing dye generally has a molar absorption coefficient as large as 10,000 to 100,000 at the hologram recording wavelength. Therefore, for attaining a high transmittance at such film thickness, an amount of addition of the sensitizing dye has to be maintained at an extremely low level, thus resulting in a significant loss in the sensitivity.